Unless your hull is close to an inch thick, of solid fiberglass, you're going to need backing plates for the seacocks. Backing plates are designed and used to distribute any stress loads over a larger footprint and to reduce and minimize any flexing of the hull at the seacock.
While most boat yards and DIY boaters use wood, because it's quick and easy, there really is a more permanent method and that is fiberglass the same material your hull is made of. Of course if you have a beautiful old wooden boat don't use fiberglass.. Many wood backing plates can still eventually absorb moisture and then swell and contract, over time, and possibly compromise the fit of the seacocks and the water tight nature. Most wood's are also far softer than solid fiberglass so it can flex some with the constant opening and closing of a seacock. Many folks swear by coating the wood with epoxy, which I've done over the years, but, in my experience, it really does not take that much more effort to make them out of fiberglass.
I now use fiberglass for my backing plates so these instructions show how to do that. If you want to use wood just replace the "epoxy" or "kitty hair" step with a product like Sikaflex 291 or 3M 4200 and secure the wood to the hull with that.
Fiberglass backing plates are easy to make but yes they are admittedly a little more time consuming. The benefit is that once you have glassed these into the boat they will last a lifetime and never need replacing even if you replace the seacocks ten+ years down the road. My motto is "any job worth doing is a job worth doing right" Morris Yachts for instance, one of the highest quality builders in the United States, currently uses solid fiberglass for their seacock installations on both their cored and solid fiberglass hulls. I personally feel wooden backing plates are a compromise but please do remember what you paid for this opinion when making your own decisions.
Fiberglass Backing Plates:
To make solid fiberglass backing plates you will need the following items.
1- Fiberglass laminating resin & hardener + some finishing or waxed resin.
2- Fiberglass in both 18oz "Woven Roving" and a standard 6oz "Fiberglass Cloth".
3- A 24 X 24 inch piece of relatively thick glass.
4- A Fiberglass roller.
5- Mold release wax for the glass.
To make the backing plate board wax the glass and begin laying up, wetting out, rolling & alternating the layers of roving and cloth until you have a fiberglass board at least 5/8 of an inch thick. The whole laminating process may take you 30 minutes. The finishing resin goes on as the last coat so it does not dry tacky.
You may also be able to find scrap pieces fiberglass around your boat yard. This would save some time instead of laminating the backing plates yourself. Even two thicknesses of thinner fiberglass epoxied together is less work than actually doing the laminating. So ask around at the yard you never know what you might find.
O.K. I've had numerous questions about the use of the plastic marine lumber Starboard and it's many "knock off" iterations. Starboard is NOT a suitable product for a backing plate.
#1 Starboard is far to flexible to offer any sort of rigid stiffening to the hull.
#2 The manufacturer states that it does NOT stick well to any adhesives which includes bedding compounds such as 5200, 4200 or Sikaflex.
#3 The concept of a backing plate is to become one with the hull, make it thicker and add stiffness so the seal between the hull and through hull is not compromised and does not flex or move. Starboard fails in all these areas..
Starboard is a great product for many uses just not one where bonding to another substrate is required or rigidity is required. Use Starboard or other similar products at your own risk.